feat: pessimistic locking for offer editing

When user A opens an offer, a lock is acquired (locked_by + locked_at).
User B opening the same offer sees a warning banner and the form is
read-only. Lock expires after 5 minutes without heartbeat.

Backend:
- POST /:id/lock — acquire lock (returns 423 if locked by another)
- POST /:id/heartbeat — refresh lock timestamp (every 2 min)
- POST /:id/unlock — release lock
- GET /:id — includes locked_by info
- PUT /:id — auto-releases lock on save

Frontend:
- Acquires lock on page load (edit mode only)
- Sends heartbeat every 2 minutes
- Releases lock on page unmount (navigate away)
- Shows warning banner with locker's name
- All inputs read-only + action buttons hidden when locked

Migration: adds locked_by (INT) and locked_at (DATETIME) to quotations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

prisma/schema.prisma

@@ -380,6 +380,8 @@ model quotations {
   status             String            @default("active") @db.VarChar(20)
   scope_title        String?           @db.VarChar(500)
   scope_description  String?           @db.Text
+  locked_by          Int?
+  locked_at          DateTime?         @db.DateTime(0)
   uuid               String?           @db.VarChar(36)
   modified_at        DateTime?         @db.DateTime(0)
   sync_version       Int?              @default(0)